1. Field of Invention
The present invention relates to lithographic projection apparatus and to device manufacturing methods. The present invention also relates to an angular encoder.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one, or several, dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In known lithographic projection apparatus that carry out scanning exposures, the positions of the mask stage and the substrate stage are measured relative to the projection system (lens) in order to control the stage positions during the scan. In practice, the positions are measured using interferometers mounted on a rigid reference frame to which the projection lens system is rigidly connected. However, in a proposed lithographic projection apparatus using EUV radiation as the exposure radiation the projection system, a mirror system with positive magnification is compliantly mounted in six degrees of freedom to the reference frame. Otherwise, the requirements on the lens dynamics would be too onerous. The same applies in other types of lithography apparatus. It is therefore necessary to measure the position of the projection system relative to the reference frame, as well as the position of the tables relative to the reference frame. A convenient arrangement to do this is required.